1. Field of the Invention
This invention pertains to a method and apparatus for forming a relief structure from a photosensitive element, and particularly to a method and apparatus for thermally developing a photosensitive element containing a composition layer capable of being partially liquefied.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a layer of the photopolymerizable composition interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photopolymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. However, developing systems that treat the element with a solution are time consuming since drying for an extended period (0.5 to 24 hours) is necessary to remove absorbed developer solution.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into the absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 describes an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element. The apparatus includes a preheating drum that is coated with a silicone rubber composition layer having a Shore A hardness rating between about 30 and 60. The rubber coating may be impregnated with aluminum particles. U.S. Pat. No. 6,797,454 B1 also describes a method and apparatus for thermally processing a photosensitive element. The apparatus includes a drum coated with a rubber composition and/or a thin tackification coating layer on a smooth metallic surface of the drum. The rubber coating provides a resilient surface that can deflect under the pressure exerted by a developing hot roller. The tackification layer temporarily adheres the photosensitive layer to the drum as the drum rotates during processing, and if no rubber layer is present, can improve the thermal conductivity between the drum and the photosensitive element.
In an embodiment of a thermal processor, commercially identified as CYREL® FAST 1000TD processor, an exterior surface of the drum includes a silicone rubber layer having a Shore A hardness of 50, and a tackification layer on the rubber layer. The tackification layer is DOW 236, a silicone dispersion in solvent, available from Dow Corning. The outermost surface of the drum may have some minimal level of roughness which forms as the coating of the tackification layer dries. But oftentimes the surface roughness is not consistently present, and even when present, the surface is insufficient to provide the desired conditions for the photosensitive element during thermal processing.
A problem arises in these thermal development processors in that the drum must have sufficient tack to hold the photosensitive element to the drum as the drum rotates, but yet allow for air trapped underneath the element to escape. In commercial embodiments of the photosensitive element, the support typically is a polymeric film, such as a polyethylene terephthalate film, having a relatively smooth surface. The support of the element resides on the drum surface. When the support contacts the tacky surface of the drum during mounting of the element, pockets of entrapped air can be formed between the relatively smooth surfaces of the element and the drum. Air trapped between the element and the drum can cause overheating of the element during thermal processing, particularly the support, which can induce defects in the resulting relief element. Areas of the support that are in contact with the drum surface are able to dissipate the heat from the element to the drum, whereas areas of the support that have an underlying pocket of air, i.e., do not contact the drum surface, and overheat. If during thermal processing, the support is at a temperature higher than the glass transition temperature due to the overheated support areas, the dimensional stability of the support can be compromised and can uncontrollably distort.
Conventional photosensitive elements having a support with a matte layer or surface that is adjacent the drum surface may be used to aid in the removal of trapped air. Generally however the matte support is not sufficiently rough enough to overcome the entrapment of air during mounting to the drum, particularly for drums with resilient outer layer. Even if the roughness of the matte surface of the support was increased to reduce air entrapment, for example, by increasing the size or density of matte particles in the matte layer, end-use procedures for the photosensitive element, such as exposure through the support forming the floor and adhesion of the plate on press, could be significantly affected.
Yet the drum surface should have sufficient tack to hold the photosensitive element to the drum during thermal development. Insufficient tackiness of the tacky layer will not retain the photosensitive element in place as the drum rotates the element through the developing cycle of heating the element and contacting with the absorbent material. If the photosensitive element is not appreciably held in contact with the drum, the element can rub against other parts of the processor and damage the relatively soft printing surface. Depending on the orientation of heating elements, the photosensitive element can possibly contact hot surfaces of the heating elements, resulting in damage to the photosensitive element, or the heating elements, or both.
During thermal treatment, the photosensitive element can lift off or sag from the drum surface due to insufficient tack on the drum surface or from the trapped air pockets that can result in uncontrolled separation between the element and the drum. Uncontrolled lifting or sagging of the photosensitive element while the element is still hot, weakens the element due to local heating and loss of support from the drum and can induce strains in the structure of the element which creates a defect, called waves, in the resulting printing relief element. The non-uniform strains imparted in the element while the support is at a temperature higher than the glass transition temperature result in deformations that remain after the element has cooled or returned to room temperature. The deformations are waves of localized distortions resulting in a non-planar topography of the photosensitive element. Because of entrapped air pockets and the uncontrolled nature of the web separation in thermal development of the prior art, waves of distortions can form in different locations in each element processed. Additionally, areas of the photosensitive element that have lifted or sagged from the drum can be further distorted when they pass through the nip resulting in permanent creases in the support layer.
Relief printing forms having waves and/or creases result in poor print performance. In multicolor printing, when one or more of the relief printing forms have waves the printed image has poor registration. Even in single color printing, waves in the relief printing form may print an image that is not an accurate reproduction of its original, so called image infidelity, by printing straight lines as curves for example. Further, the relief printing form having waves may incompletely print the image due to intermittent contact of the inked surface of the printing form to the printed substrate.